Transistors can be divided into two main types: bipolar junction transistors and field-effect transistors. Both types share a common structure comprising three electrodes with a semi-conductive material disposed therebetween in a channel region. The three electrodes of a bipolar junction transistor are known as the emitter, collector and base, whereas in a field-effect transistor the three electrodes are known as the source, drain and gate. Bipolar junction transistors may be described as current-operated devices as the current between the emitter and collector is controlled by the current flowing between the base and emitter. In contrast, field-effect transistors may be described as voltage-operated devices as the current flowing between source and drain is controlled by the voltage between the gate and the source.
Transistors can also be classified as p-type and n-type according to whether they comprise semi-conductive material which conducts positive charge carriers (holes) or negative charge carriers (electrons) respectively. The semi-conductive material may be selected according to its ability to accept, conduct, and donate charge. The ability of the semi-conductive material to accept, conduct, and donate holes or electrons can be enhanced by doping the material. The material used for the source and drain electrodes can also be selected according to its ability to accept and injecting holes or electrodes.
For example, a p-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating holes, and selecting a material for the source and drain electrodes which is efficient at injecting and accepting holes from the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the HOMO level of the semi-conductive material can enhance hole injection and acceptance. In contrast, an n-type transistor device can be formed by selecting a semi-conductive material which is efficient at accepting, conducting, and donating electrons, and selecting a material for the source and drain electrodes which is efficient at injecting electrons into, and accepting electrons from, the semi-conductive material. Good energy-level matching of the Fermi-level in the electrodes with the LUMO level of the semi-conductive material can enhance electron injection and acceptance.
Transistors can be formed by depositing the components in thin films to form a thin film transistor (TFT). When an organic material is used as the semi-conductive material in such a device, it is known as an organic thin film transistor (OTFT). OTFTs may be manufactured by low cost, low temperature methods such as solution processing. Moreover, OTFTs are compatible with flexible plastic substrates, offering the prospect of large-scale manufacture of OTFTs on flexible substrates in a roll-to-roll process.
Various arrangements for organic thin film transistors are known. One such device is an insulated gate field-effect transistor which comprises source and drain electrodes with a semi-conductive material disposed therebetween in a channel region, a gate electrode disposed adjacent the semi-conductive material and a layer of insulating material disposed between the gate electrode and the semi-conductive material in the channel region.
One use of transistors is in active matrix optical devices such as light-detecting and light emitting devices, in particular organic light-emissive devices and organic photodetector arrays. For example, an active matrix organic light-emissive display comprises a matrix of organic light-emissive devices forming the pixels of the display. Each organic light emissive device comprises an anode, a cathode, and an organic light-emissive layer disposed therebetween. In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic light-emissive layer to form an exciton which then undergoes radiative decay to give light (in light detecting devices this process essentially runs in reverse). Other layers may also be provided between the electrodes in order to enhance charge injection and transport such as a hole injecting layer, an electron injecting layer, a hole transporting layer, and/or an electron transporting layer. Blends of materials can also be used in order to enhance operation, such as a blend of charge transporting and emissive material. An organic photoresponsive device comprises the same structure of an organic layer located between two electrodes, and can in fact be regarded as an organic light-emitting device operating in reverse (i.e. holes and electrons are generated and separated upon exposure of the device to light).
The pixels of an active matrix organic light-emissive display can be switched between emitting and non-emitting states by altering the current flow through them using a memory element typically comprising a storage capacitor and two transistors, one of which is a driving transistor.
Use of a common substrate for thin film transistors and organic light-emissive devices in order to form an active matrix organic light-emissive display is known. For example, U.S. Pat. No. 6,150,668 discloses depositing organic thin film transistors (OTFTs) and organic light-emissive devices (OLEDs) on a common substrate and using the same layer of material for both the OTFT gate and the OLED anode. The OLED cathode is selectively deposited through a shadow mask. Furthermore, U.S. Pat. No. 6,924,503 discloses depositing oTFTs and OLEDs on a common substrate and using the same layer of material for the source and drain of the OTFT and the anode of the OLED. This document also discloses formation of a top-gate of the OTFT and cathode of the OLED in one step by depositing a metal over the whole surface and then patterning the layer to form the top gate and the cathode.
In light of the above, it is evident that in the prior art monolithic OLED/OTFT constructions, some of the layers in the OLED and OTFT must be selectively deposited and patterned by post-deposition treatment. For example, separate structures are provided for containing the organic semi-conductive material of the OTFT and the organic light-emissive material of the OLED. Furthermore, in the prior art arrangements, the cathode of the OLED and the gate of the OTFT have either been selectively deposited or patterned by post-deposition treatment in order to prevent electrical shorts between the OTFT and OLED on a top side of the device.
It is an aim of certain embodiments of the present invention to provide methods of manufacturing active matrix organic light-emissive displays comprising thin film transistors and organic light-emissive devices deposited on a common substrate which are easier and quicker than prior art arrangements thus saving time and cost in the display manufacturing process.
It is further aim of certain embodiments of the present invention to reduce processing steps involved in such methods and produce new structures for active matrix organic light-emissive displays comprising thin film transistors and organic light-emissive devices deposited on a common substrate.
It is a further aim of certain embodiments of the present invention to provide alternative methods and structures for isolating thin film transistors and organic light-emissive devices deposited on a common substrate in an active matrix organic light-emissive display to prevent electrical shorts between the thin film transistors and organic light-emissive devices.
It is a further aim of certain embodiments of the present invention to provide alternative methods and structures for encapsulating thin film transistors which are deposited on a common substrate with organic light-emissive devices in an active matrix organic light-emissive display.